NAME¶
SSL_CTX_add_client_custom_ext, SSL_CTX_add_server_custom_ext - custom TLS
extension handling
SYNOPSIS¶
#include <openssl/ssl.h>
int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
custom_ext_add_cb add_cb,
custom_ext_free_cb free_cb, void *add_arg,
custom_ext_parse_cb parse_cb,
void *parse_arg);
int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
custom_ext_add_cb add_cb,
custom_ext_free_cb free_cb, void *add_arg,
custom_ext_parse_cb parse_cb,
void *parse_arg);
int SSL_extension_supported(unsigned int ext_type);
typedef int (*custom_ext_add_cb)(SSL *s, unsigned int ext_type,
const unsigned char **out,
size_t *outlen, int *al,
void *add_arg);
typedef void (*custom_ext_free_cb)(SSL *s, unsigned int ext_type,
const unsigned char *out,
void *add_arg);
typedef int (*custom_ext_parse_cb)(SSL *s, unsigned int ext_type,
const unsigned char *in,
size_t inlen, int *al,
void *parse_arg);
DESCRIPTION¶
SSL_CTX_add_client_custom_ext() adds a custom extension for a TLS client
with extension type
ext_type and callbacks
add_cb,
free_cb and
parse_cb.
SSL_CTX_add_server_custom_ext() adds a custom extension for a TLS server
with extension type
ext_type and callbacks
add_cb,
free_cb and
parse_cb.
In both cases the extension type must not be handled by OpenSSL internally or an
error occurs.
SSL_extension_supported() returns 1 if the extension
ext_type is
handled internally by OpenSSL and 0 otherwise.
EXTENSION CALLBACKS¶
The callback
add_cb is called to send custom extension data to be
included in ClientHello for TLS clients or ServerHello for servers. The
ext_type parameter is set to the extension type which will be added and
add_arg to the value set when the extension handler was added.
If the application wishes to include the extension
ext_type it should set
*out to the extension data, set
*outlen to the length of the
extension data and return 1.
If the
add_cb does not wish to include the extension it must return 0.
If
add_cb returns -1 a fatal handshake error occurs using the TLS alert
value specified in
*al.
For clients (but not servers) if
add_cb is set to NULL a zero length
extension is added for
ext_type.
For clients every registered
add_cb is always called to see if the
application wishes to add an extension to ClientHello.
For servers every registered
add_cb is called once if and only if the
corresponding extension was received in ClientHello to see if the application
wishes to add the extension to ServerHello. That is, if no corresponding
extension was received in ClientHello then
add_cb will not be called.
If an extension is added (that is
add_cb returns 1)
free_cb is
called (if it is set) with the value of
out set by the add callback. It
can be used to free up any dynamic extension data set by
add_cb. Since
out is constant (to permit use of constant data in
add_cb)
applications may need to cast away const to free the data.
The callback
parse_cb receives data for TLS extensions. For TLS clients
the extension data will come from ServerHello and for TLS servers it will come
from ClientHello.
The extension data consists of
inlen bytes in the buffer
in for
the extension
extension_type.
If the
parse_cb considers the extension data acceptable it must return 1.
If it returns 0 or a negative value a fatal handshake error occurs using the
TLS alert value specified in
*al.
The buffer
in is a temporary internal buffer which will not be valid
after the callback returns.
NOTES¶
The
add_arg and
parse_arg parameters can be set to arbitrary
values which will be passed to the corresponding callbacks. They can, for
example, be used to store the extension data received in a convenient
structure or pass the extension data to be added or freed when adding
extensions.
The
ext_type parameter corresponds to the
extension_type field of
RFC5246 et al. It is
not a NID.
If the same custom extension type is received multiple times a fatal
decode_error alert is sent and the handshake aborts. If a custom
extension is received in ServerHello which was not sent in ClientHello a fatal
unsupported_extension alert is sent and the handshake is aborted. The
ServerHello
add_cb callback is only called if the corresponding
extension was received in ClientHello. This is compliant with the TLS
specifications. This behaviour ensures that each callback is called at most
once and that an application can never send unsolicited extensions.
RETURN VALUES¶
SSL_CTX_add_client_custom_ext() and
SSL_CTX_add_server_custom_ext() return 1 for success and 0 for failure.
A failure can occur if an attempt is made to add the same
ext_type more
than once, if an attempt is made to use an extension type handled internally
by OpenSSL or if an internal error occurs (for example a memory allocation
failure).
SSL_extension_supported() returns 1 if the extension
ext_type is
handled internally by OpenSSL and 0 otherwise.